I was taught by a skilled machinest long ago to first drill with a
drill bit that is the same size or slightly bigger than the web of the
drill needed for the final hole. I think this is what Robin means by
the 15-30%. If you are going to drill a 5 mm hole then use a 1 mm (
20% of 5mm ) pilot drill.

But for holes up to about 5 mm I usually don't use a pilot drill. But
for a 25 mm hole, a 5 mm pilot hole helps me. Try it and make up
your own mind.

Dan




oEmails (Dave Baker) wrote in message

No one suggested predrilling with a smaller diameter drill.


I work by 15-30% (for a new drill the web is typically about 15% of the
diameter).

Don't understand that comment.